Chevron Corp. on Monday accused lawyers suing the company in Ecuador of submitting falsified oil-field contamination reports to the court, the latest serious allegation in the long-running $27 billion lawsuit.

The reports in question showed dangerous levels of contamination at two former drilling sites in the Ecuadoran rain forest and bore the signature of an American biologist working on behalf of the plaintiffs' lawyers.

But in a recent deposition, the biologist, who is no longer involved in the case, said he never concluded that the drilling sites were seriously polluted and posed a health risk. The reports with his signature, he said in the deposition, must have been written by someone else.

"I concluded that I did not see significant contamination that posed immediate threat to the environment or to humans or wildlife around it," said biologist Charles Calmbacher, according to the deposition. Chevron released the deposition on Monday.

Representatives for the plaintiffs expressed surprise at Calmbacher's statements and said there had been no attempt to falsify reports.

Monday's allegations could further complicate an already complex and bitter trial.

Residents of the Ecuadoran Amazon are suing Chevron over oil-field pollution that they say has sickened and killed people in the region. Texaco used to drill for oil there in partnership with the state-run oil company Petroecuador, and Chevron bought Texaco in 2001.

Chevron, based in San Ramon, considers the case a shakedown.

When Texaco pulled out of Ecuador in the early 1990s, the company agreed to clean up some of the oil drilling sites while leaving the rest to Petroecuador, which continued pumping oil in the region. Any remaining pollution problems, Chevron executives say, are Petroecuador's responsibility.

Chevron has often complained that the trial, held in the small Ecuadoran town of Lago Agrio, has been tainted by political interference and possible corruption.

"This was wholesale fabrication," Chevron spokesman Kent Robertson said of the disputed reports, which the company wants stricken from the court record. "If there were any merit to the plaintiffs' case, they wouldn't have to resort to fraud."

The new allegations address one of the key issues in the trial - Texaco's cleanup efforts.

The plaintiffs consider the cleanup work a sham, saying the company did little more than dump dirt on pools of oil and hazardous chemicals. In 2004, Calmbacher examined four of the former drilling sites. The plaintiffs' legal team submitted two reports with his signature in 2005.

But in a deposition recorded last week, Calmbacher said the reports submitted to the court did not match his findings. He had given the legal team signed signature pages with the understanding that they would be attached to a Spanish-language translation of his work, which he said did find petroleum hydrocarbons at the site but not at dangerous levels.

"I did not reach these conclusions, and I did not write this report," he said in the deposition.

Hinton said Calmbacher had been working closely with the plaintiffs' legal team and would have seen any reports submitted with his name. She also noted that in a 2004 New York Times story, Calmbacher had compared Chevron's defense in the case to tobacco companies denying a link between smoking and lung cancer.

"While we take Dr. Charles Calmbacher's statements about the reports seriously, we believe his recollection almost six years after the fact is inaccurate," Hinton said.